This invention relates to an image display apparatus, such as a display using a cathode ray tube (CRT) or a television receiver, and, more particularly, to an image display apparatus having a deflection circuit having a function for correcting a rotational distortion of an image frame on the CRT.
With the development of microprocessors, many image display apparatuses such as CRT displays have been used in the field of CAD, CAM, work stations and the like. There is now an increased demand for CRT display apparatuses having improved accuracy, in particular for those which are used to process diagrams and drawings correctly and accurately, and there is a need to display an image in a predetermined correct position.
An image display apparatus using a CRT is constructed in such a manner that the CRT is fixed by being mounted on a rectangular frame, and a rectangular facing frame called a bezel is attached at a front outer edge of the CRT. Images are ordinarily displayed in a rectangular display area (raster area) set inside the bezel frame.
If, at the time of assembly of such a display screen portion, the CRT and the bezel are mounted without being correctly positioned relative to each other in accordance with design criteria, the display area is shifted from the bezel frame in a horizontal or vertical direction or is rotated relative to the bezel frame. In such a case, there is the problem of a displayed content in a peripheral portion of the display area being hidden behind the bezel or the problem of a feeling of unnaturalness in viewing a displayed image.
Even after the display apparatus has been assembled, an error in positioning the display area in a horizontal or vertical direction can easily be corrected by a method conventionally practiced, i.e., a method of adjusting a DC component of a deflection current through the horizontal or vertical deflection coil so that the center of the display area coincides with the center of the bezel frame.
However, a phenomenon in which the display area is rotated relative to the bezel frame (hereinafter referred to as rotational distortion) cannot be corrected easily. That is, after the assembly of the image display apparatus, the CRT is energized to display an image to confirm the existence of a rotational distortion and the degree of distortion. Next, the image display is temporarily put off and the bezel attachment screws are loosened to adjust the mount angle. Thereafter, the screws are turned to fasten the bezel and the CRT is energized again to display the image and to check the rotational distortion. It is necessary to repeat these operations to correct the distortion, and the efficiency of this process is very low.
There is a possibility of rotational distortions due to an error in setting the deflection yoke mount angle and the influence of earth magnetism, as well as the above-mentioned distortion due to an error in the relative position of the CRT and the bezel. Further, rotational distortion can be caused as rotation of only one of the horizontal and the vertical axes of the display area, i.e., a parallellogramatic distortion, as well as rotation the display area as a whole.
A method is disclosed in Japanese Utility Model Laid-Open Publication Hei 2-67549 as a method correcting such rotational distortions in a simple manner. In this method, coils for removing rotational distortions are provided on a deflection yoke, and currents flowing through the coils are controlled to correct rotational distortions.
However, this method requires a deflection yoke provided with special rotational distortion correcting coils and, therefore, increases the manufacture cost of the display considerably.
Similar problems are also encountered in the case of television receivers. That is, a displayed content in a peripheral portion of the display area may be hidden behind the bezel or an axis of an image to be displayed horizontally or vertically, and the image may be inclined so that the viewer views the image as being unnatural.